Holt McDougal Algebra 1 Solving Systems by Graphing Warm Up Evaluate each expression for x = 1 and y...

Holt McDougal Algebra 1

Solving Systems by Graphing

Warm UpEvaluate each expression for x = 1 and y =–3.

1. x – 4y 2. –2x + y

Write each expression in slope-intercept

form.

3. y – x = 1

4. 2x + 3y = 6

5. 0 = 5y + 5x

13 –5

y = x + 1

y = x + 2

y = –x



Identify solutions of linear equations in two variables.

Solve systems of linear equations in two variables by graphing.

Objectives



systems of linear equationssolution of a system of linear equations

Vocabulary



A system of linear equations is a set of two or more linear equations containing two or more variables. A solution of a system of linear equations with two variables is an ordered pair that satisfies each equation in the system. So, if an ordered pair is a solution, it will make both equations true.



Tell whether the ordered pair is a solution of the given system.

Example 1A: Identifying Solutions of Systems

(5, 2);

The ordered pair (5, 2) makes both equations true.(5, 2) is the solution of the system.

Substitute 5 for x and 2 for y in each equation in the system.

3x – y = 13

2 – 2 00 0

0 3(5) – 2 13

15 – 2 13

13 13

3x – y =13



Example 1B: Identifying Solutions of Systems


(–2, 2);x + 3y = 4–x + y = 2

–2 + 3(2) 4

x + 3y = 4

–2 + 6 44 4

–x + y = 2

–(–2) + 2 24 2

Substitute –2 for x and 2 for y in each equation in the system.

The ordered pair (–2, 2) makes one equation true but not the other.

(–2, 2) is not a solution of the system.



All solutions of a linear equation are on its graph. To find a solution of a system of linear equations, you need a point that each line has in common. In other words, you need their point of intersection.

y = 2x – 1

y = –x + 5

The point (2, 3) is where the two lines intersect and is a solution of both equations, so (2, 3) is the solution of the systems.



Solve the system by graphing. Check your answer.Example 2A: Solving a System by Graphing

y = xy = –2x – 3 Graph the system.

The solution appears to be at (–1, –1).

The solution is (–1, –1).

CheckSubstitute (–1, –1) into the system.

y = x

y = –2x – 3

• (–1, –1)

y = x

(–1) (–1)

–1 –1

y = –2x – 3

(–1) –2(–1) –3

–1 2 – 3–1 – 1



Solve the system by graphing. Check your answer.Check It Out! Example 2a

y = –2x – 1 y = x + 5 Graph the system.

The solution appears to be (–2, 3).

Check Substitute (–2, 3) into the system.

y = x + 5

3 –2 + 5

3 3

y = –2x – 1

3 –2(–2) – 1

3 4 – 1

3 3The solution is (–2, 3).

y = x + 5

y = –2x – 1



Example 3: Problem-Solving Application

Wren and Jenni are reading the same book. Wren is on page 14 and reads 2 pages every night. Jenni is on page 6 and reads 3 pages every night. After how many nights will they have read the same number of pages? How many pages will that be?



11 Make a Plan

Write a system of equations, one equation to represent the number of pages read by each girl. Let x be the number of nights and y be the total pages read.

Totalpages is

number read

everynight plus

already read.

Wren y = 2 x + 14

Jenni y = 3 x + 6

Example 3 Continued



Solve22

Example 3 Continued

(8, 30)

Nights

Graph y = 2x + 14 and y = 3x + 6. The lines appear to intersect at (8, 30). So, the number of pages read will be the same at 8 nights with a total of 30 pages.



Look Back33

Check (8, 30) using both equations.

Number of days for Wren to read 30 pages.

Number of days for Jenni to read 30 pages.

3(8) + 6 = 24 + 6 = 30

2(8) + 14 = 16 + 14 = 30

Example 3 Continued



Check It Out! Example 3

Video club A charges $10 for membership and $3 per movie rental. Video club B charges $15 for membership and $2 per movie rental. For how many movie rentals will the cost be the same at both video clubs? What is that cost?



11 Make a Plan

Write a system of equations, one equation to represent the cost of Club A and one for Club B. Let x be the number of movies rented and y the total cost.

Totalcost is price

for eachrental plus

member-ship fee.

Club A y = 3 x + 10

Club B y = 2 x + 15

Check It Out! Example 3 Continued



Solve22

Graph y = 3x + 10 and y = 2x + 15. The lines appear to intersect at (5, 25). So, the cost will be the same for 5 rentals and the total cost will be $25.




Look Back33

Check (5, 25) using both equations.

Number of movie rentals for Club A to reach $25:

Number of movie rentals for Club B to reach $25:

2(5) + 15 = 10 + 15 = 25

3(5) + 10 = 15 + 10 = 25




Lesson Quiz: Part I


1. (–3, 1);

2. (2, –4);

yes

no



Lesson Quiz: Part II

Solve the system by graphing.

3.

4. Joy has 5 collectable stamps and will buy 2 more each month. Ronald has 25 collectable stamps and will sell 3 each month. After how many months will they have the same number of stamps? How many will that be?

(2, 5)

4 months

y + 2x = 9

y = 4x – 3

13 stamps



1)Must fold2)Must have color3)Must write out the problem and

include somewhere in the organizer

4)Must show all 3 methods of solving the problem in the organizer

(Hint: All 3 answers should be the same!)

Holt McDougal Algebra 1 Solving Systems by Graphing Warm Up Evaluate each expression for x = 1 and y...

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